The present invention relates to the field of optical correlators and more particularly to joint transform correlators.
In a conventional joint transform correlator (JTC), the input signal to be identified, often a two dimensional image of a scene, and the reference signal to be compared to the input signal, are displayed in the same plane. The output of the JTC contains three terms. One is the sum of the autocorrelation of the input scene and the auto-correlation of the reference signal that is formed on the optical axis (DC terms). The other two terms are the cross-correlation functions between the input signal and the reference signal that are diffracted off-axis. These two cross-correlation terms are positioned away from the optical axis and are symmetrical thereto. Either one indicates the presence and the locations of identified input targets. Conventional JTC are well known in the art. U.S. Pat. No. 4,695,973, issued to Francis T.S. Yu, describes a JTC having structure quite similar to the embodiments of the present invention. U.S. Pat. No. 4,832,447 issued to Bahram Javidi describes a JTC which employs a non-linear spatial light modulator (SLM) at the Fourier plane and which can be advantageously used in implementing the present invention.
Ideally a correlation peak would occur in the JTC output plane only at the location of each target in the input scene. However, if there are multiple targets in the input scene, it is well known that spurious correlation signals can be generated among these multiple targets themselves, each correlating with one another. One example of this is a scene with many prominent repetitions of a feature, such as a picture of a house with a picket fence. This causes false alarms and degrades the performance and reliability of the JTC.